None
Not applicable
1. Field of the Invention
The preferred embodiments are directed generally to alternating current (AC) to direct current (DC) switching power supplies or power conversion units employing power factor correction circuitry. More particularly, the preferred embodiments are directed to AC to DC switching power supplies having an integrated rectifying bridge and boost control switch for power factor correction.
2. Background of the Invention
FIG. 1 shows a schematic diagram of a switching power supply of the related art having an alternating current source 2 coupled to a full wave rectifier bridge 4 comprising four diodes 6, 8, 10 and 12. The bridge 4 converts the alternating current (AC) power signal from the source 2 into a direct current (DC) waveform having a ripple at twice the source frequency. In systems where the source 2 has a standard 120 volt root means square (rms) voltage operating at 60 hertz, the rectified waveform is thus a signal having an approximately 170 volt peak and a 120 hertz ripple. If the desired output voltage of the power supply is approximately 170 volt, then all that would be needed is additional filtering circuitry to remove the ripple. However, switching power supplies such as exemplified in FIG. 1 are used in a boost configuration, meaning that the voltage applied to the load RL is higher than the peak voltage experienced in the source 2. Thus, remaining portions of the power supply boosts the voltage to the desired level.
In the related art, the combination of the inductor 14, switch 16 and diode 18 operate as a boost circuit. In particular, when switch 16 conducts, current flows through the inductor 14, switch 16, and back to the source 2. During this time, energy is stored in the magnetic field of the inductor 14. As part of the cycle, switch 16 opens and the collapsing magnetic field of the inductor 14 creates a voltage that forward biases the diode 18. Thus, during the period when the switch 16 is not conducting, current flows through the inductor 14, diode 18 and on to the load RL. In the related art, the frequency at which the switch is opened and closed is 50 to 100 kilohertz. Moreover, the duty cycle of the signal applied to the switch 16 controls the charging and discharge time of the inductor, and therefore controls the voltages and current levels supplied to the load or RL. Most related art power supplies also implement a power factor correction (PFC) system where current from the source 2 flows through the inductor 14 in such a manner that the power factor (the cosine of the angle between the current supplied by the source 2 and the voltage of the source 2) is as close to unity as possible. In power factor corrected power supplies, the duty cycle of the signal applied to the switch 16 changes as a function of the instantaneous voltage of the source 2, and also the voltage and current supplied to the load.
Power supply manufacturers, especially those manufacturers who make power supplies for computer systems, are faced with continued pressure to increase the efficiency of their power supplies, while simultaneously decreasing the size. The size of a power supply is directly related to the size of the heat sink required. If the amount of heat that needs to be dissipated is lowered, there can be a corresponding decrease in heat sink size and therefore power supply size. Electrical energy converted to heat across devices such as the diodes is proportional to the current through the device.
Consider the time of the positive half cycle of the voltage source 2 and with switch 16 open, the discharge cycle of the boost inductor. During this time, traditional current flow moves through the diode 6, boost diode 18, and returns to the power source through diode 12xe2x80x94a three diode forward power loss. When switch 16 is closed, the charging cycle of the inductor 14, the current flow experiences a forward power loss associated with the two diodes in the bridge 4 and a conduction loss associated with switch 16. While the switch 16 has a significantly lower loss than the diodes, the loss may be appreciable at high currents.
Now consider the negative half cycle of the power source 2. During inductor charging, the current flow experiences the forward power loss associated with diodes 8 and 10, and a conduction loss associated with switch 16. During the discharge phase, the current flow experiences forward power loss associated with diode 8, boost diode 18 and diode 10.
There have been attempts in the related art to reduce the loss associated with devices inside the power supply. Most notable of these attempts is placing multiple switches 16 in parallel in an attempt to reduce losses across the boost switch 16. While having multiple boost switches may reduce the loss associated with that portion of the circuitry in the inductor charge cycle, this technique does not address the forward power losses of the diodes experienced during both the charging and discharge cycle of the inductor.
Thus, what is needed in the art is a system and related method to reduce power loss internal to the power supply during the rectification and voltage boost process.
The problems noted above are solved in large part by an integrated bridge and boost circuit for a power conversion unit. More particularly, in the preferred embodiments, the inductor is coupled between the alternating current (AC) source and a rectifying bridge topology where at least two of the diodes are shunted with switch devices, which switch devices in the preferred embodiments are field effect transistors (FETs). Charging cycles of the inductor in either the positive or negative half cycle of the source voltage take place with current flowing through the inductor and back to the source through two FETs. The conduction power loss associated with the two FETs is significantly less than the power loss associated with the two diodes and boost switch experienced in the related art charging cycle. During the discharge cycle of the inductor, the current flow experiences the forward power loss associated with two power diodes, rather than the three diode forward power loss associated with the related art devices.
Further, by reducing the forward power loss, and therefore the total amount of heat that must be dissipated by the heat sink of the power supply, it is possible to shrink the overall size of the power supply.
The disclosed devices and methods comprise a combination of features and advantages which enable it to overcome the deficiencies of the prior art devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings.